In a computer system such as a vehicle control computer system, control data calculated as main data are stored as backup data in a backup memory together with checkup data, so that the backup data may be later checked up by using the checkup data. For instance, in a mirror checkup method, a reverse data of the main data are stored as the checkup data and the main data is compared with the reverse data to determine whether the main data is normal.
In some instances, the main data and the checkup data cannot be written into the backup memory at the same time when the main data is updated. If the main data has a 4-byte data length and is calculated by a 32-bit microcomputer, writing of the main data and the checkup data are performed separately in two stages. That is, the main data is written first followed by writing of the checkup data. If power supply cut-off occurs between the writing of the main data and the subsequent writing of the checkup data, the main data may be determined later to be incorrect due to erroneous writing of the checkup data.
It is proposed to improve reliability of the main data by storing the main data together with two mirror data as the checkup data. In this instance, the data writing in the backup memory is performed by writing in sequence the checkup data, the main data and again the checkup data, so that the main data may not be determined to be erroneous due to the power supply cut-off between the data writing operations. However, this requires more data storage areas in the backup memory, because the mirror data also have the same data length as that of the main data.